The present invention relates to a magnetic visual display that uses magnetic force to orient magnetically active flakes contained within a dispersion medium to allow light to pass therethrough.
The existing techniques of forming a visual display through magnetic means generally comprise applying a magnetic field to fine magnetic particles dispersed within a viscous liquid. The particles migrate to the magnetic field and accumulate along the locus of the field, thereby creating an image comprising an accumulation of the particles along the locus of the magnetic field.
The attractability of these particles may be defined as an additive process, that is, prior to drawing, the entire field of visible background is generally void of any magnetic particles. When a magnetic field is displayed to the liquid, the magnetic particles are drawn up from the bottom of the liquid to the top of the liquid, thus producing a visible image at the top surface.
However, after attraction, the particles tend to precipitate away from the surface of the liquid, making it difficult to retain the image over an extended period of time. Additionally, since the magnetic particles within the influence of the magnetic field are attracted to the field, magnetic particles follow the locus of the magnetic field and are carried away from the desired area of demarcation; thus forming a discontinuous line with reduced contrast and resolution.
The prior art has dealt with contrast and resolution difficulties in a number of ways. For instance, the patent to Murata, et al. (U.S. Pat. No. 4,643,684), discloses the use of a magnetic display panel having a dispersing medium having a yield value of 5 dyne/cm.sup.2 or more, the medium comprising an inorganic thickener, fine magnetic particles, and a colorant. Murata discloses the use of a multi-cell structure which confines the dispersing medium within each cell, the structure assisting in limiting the migration of the medium and the magnetic particles from one cell into the next during the application of a magnetic field to the particles.
However, regardless of the precautions taken by the prior art, the action of the magnetic field on the magnetic particles dispersed within the liquid of the prior magnetic marking devices produces a number of inherent difficulties.
For example, during movement of the magnetic field across the magnetic particle containing liquid, the magnetic particles move through the liquid, from the bottom of the liquid to the top of the liquid, to the magnetic field. This localized movement of particles through the liquid creates a void of particles within the liquid. This void is created when the particles are pulled through to and along the top layer of the substrate by their attraction to the magnetic field. When the magnetic field is moved, as when the device is used for drawing purposes, the attracted particles are pulled along the locus of the magnetic field, throughout the substrate, creating an incomplete distribution of particles.
Additionally, a magnetic field is required to erase the image produced by these prior art devices. The erasing magnet repositions the magnetic particles after magnetic field attraction. Thus, when the cleaning or erasure of a display is desired, a magnetic field is applied to the bottom of the device to draw the magnetic particles from the top of the liquid to their original position at the bottom of the liquid, thus eliminating the image-producing particles from the top of the liquid. However, there exist a number of limitations of this technique of erasure. For instance, incomplete or nonuniform application of the magnetic field across the bottom of the liquid produces localized areas of particle accumulation after erasure, thus preventing the subsequent drawing of a true line during application of the magnetic field to the top of the liquid due to the incomplete distribution of particles throughout the liquid. Additionally, after repeated use and erasure by magnetic means, it becomes extremely difficult to redisperse the particles to attain uniformity throughout the liquid due to the magnetically attractive properties of the particles. Thus, there exists a need for an apparatus and method for producing a magnetic display which eliminates the drawing and erasure difficulties inherent in the additive processes used in the prior art magnetic display devices.
The present invention provides a magnetic visual display which is true, uniform, and of high resolution and contrast. The present invention also provides a method and apparatus for producing an image by orienting magnetically active flakes contained within a dispersion medium such that when a magnetic field is displayed to the flakes within the dispersion medium, the magnetically active flakes are oriented to change the light transmission characteristics of the dispersion medium. The orientation of the magnetically active flakes of the present invention occurs without gross translation of the flakes within the dispersion medium, thus providing a uniform, consistent dispersion of the flakes throughout the medium.